The invention relates to a method for operating at least two lifting gears in a group operation, wherein each lifting gear has a hoist, via which a respective load picking-up means can be raised by a lifting procedure or can be lowered by a lowering procedure, in which group operation, in a synchronous operating mode, firstly a common lifting procedure is carried out by means of the at least two hoists in order to move a load fastened to the load picking-up means, after the common lifting procedure, for changing from the synchronous operation to an individual operation or a multi-operation, at least one of the hoists involved in the common lifting procedure is deactivated, and at least one of the involved hoists remains activated such that only each activated hoist can carry out a lifting procedure or lowering procedure relative to each deactivated hoist.
The invention also relates to an assembly comprising at least two lifting gears.
German laid-open document DE 10 2011 053 014 A1 describes a method for operating at least two lifting gears in a normal operation and in a group operation. Within the group operation, a tandem operation is possible, in which via a single control switch the hoists of both cranes can be operated synchronously and in this case can also be moved horizontally. Within the group operation, it is also possible to perform an individual operation, in which the cranes and in particular their hoists and load picking-up means can be moved individually relative to one another.
The tandem operation can cause critical operating states, such as e.g. an oblique position of the load, which require an individual operation for this to be rectified. Since in this case the distribution of the load to the hoists and load picking-up means supporting the load is changed, this can result in the hoist, which is deactivated for the individual operation, becoming overloaded and consequently can result in a load crashing down.
It is known that hoists have an overload safety device comprising a load sensor which determines a load value corresponding to a load acting upon the hoist during a lifting procedure, the determined load value is compared to a permissible threshold value and the lifting procedure is prevented if the determined load value exceeds the threshold value. However, such an overload safety device which detects and monitors hoists which are active merely during a lifting procedure, cannot detect the overloading of a deactivated hoist and cannot prevent an imminent crash of the load.
It is known from German laid-open document DE 31 47 158 A1 to prevent an oblique position of the load in the case of two crane hoists operated in a tandem operation. For this purpose, a sensor is arranged on a load cross-beam which is suspended on load picking-up means of the two crane hoists, said sensor being used to determine and evaluate the position of the load cross-beam. Since a hoist which is deactivated after the tandem operation can be overloaded even without a detectable oblique position of the load, an imminent crash of the load also cannot be reliably prevented by such a sensor.
It is known from German load-open document DE 10 2006 040 782 A1 to operate two rotary luffing cranes in each case in a normal operation and in this case to coordinate their mutually independent movements in order to be able to move a common load with both cranes. This is referred to as tandem operation in said document. A synchronous operation of the two cranes which is effected via a single control switch is not performed in this case. Instead, in a manner which is typical of a normal operation the controllers of the two cranes are active so that the crane operator of each crane can instigate movements of his crane. In this case, the overload safety devices of both cranes are integrated in order to ensure that movements of the first crane do not cause any overload states in the other crane. The movements of the rotary mechanism and of the luffing mechanism which are instigated by the respective crane operators in this coordinated normal operation are not carried out if one of the overload safety devices detects an overload. It is also described that the controller of one crane can be deactivated and the rotary mechanism and the luffing mechanism of this crane can be controlled from another crane.